It's not really an opinion ... it's a statement of the rules
"The thief’s damage-reduction ability can be used again each round until he is eliminated. If there are multiple thieves in the party, their abilities are cumulative. The ability only works on damage, however. If the trap has other effects, the thief can not prevent them."
Rulebook, p.4, "Combat" And it then follows it with the exact same example as you are asking.
